This invention relates to the characterization of optical components and more specifically to a process and an apparatus for measuring polarization dispersion in single-mode optical fibers.
Polarization dispersion (commonly known in the literature as PMD from the initials of the English denomination xe2x80x9cPolarization Mode Dispersionxe2x80x9d) is defined as the differential group delay between two orthogonally polarized modes, which causes digital communication system impulses to widen or analog communication system impulses to distort. This phenomenon is due to fiber manufacturing imperfections (elliptical nucleus and material anisotropy) or to external mechanical stresses (flattening, folding, torsion and the like) that make propagating light divide into two local polarization modes that propagate at different speeds. Unless properly controlled, this phenomenon can limit the performance of high capacity (10 Gbit/s or more) optical systems or even inhibit operation.
Dispersion of the polarization of the fibers and components with which new high capacity optical networks are manufactured must therefore be accurately measured or characterized in terms of polarization dispersion in view of the installation of new systems and where existing systems are being enlarged.
A number of techniques for measuring polarization dispersion have been the subject of international regulations. In particular, Recommendation ITU-T G.650 proposes using analysis of the Jones matrix self-values, the Poincarxc3xa9 sphere and the fixed polarizer and interferometry methods. All these techniques enable the measurement of the differential delay of the average group, namely of the magnitude that best characterizes polarization dispersion of optical fibers and its influence on transmission systems.
All the measurement methods mentioned above have the drawback of requiring access to both fiber ends. These techniques in fact analyze the signal transmitted by the optical signal under test and thus require the use of one end for launching the test optical signal and the other end for measuring the variations induced by the fiber on the signal polarization features. This limitation is clearly irrelevant for laboratory or factory measurements on optical fibers and cables wound on coils (where the two ends are at a maximum distance of a few meters) or on optical fiber components, but makes polarization dispersion measurement difficult on the fibers making up the transmission lines of already installed optical systems where the opposite ends of the optical fibers can be placed at a distance of many tens of kilometers. In these cases, at least two operators connected to one another are required; one for checking the optical source and the other for checking the polarization analyzer, or connection must be made between the two instruments for measurement control transmission. Both solutions are complicated and costly.
Techniques for measuring polarization dispersion based on the analysis of the signal reflected from the far end of the optical fiber have already been proposed, requiring access to one end only of the optical fiber being measured, with remarkable time and resource savings.
In particular, the article xe2x80x9cSingle-End Polarization Mode Dispersion Measurement Using Backreflected Spectra Through A Linear Polarizesxe2x80x9d in the Journal of Lightwave Technology, Volume 17, Number 10 of October 1999, pages 1835-1842, by the Inventors, describes an apparatus whose light issued by a tuneable source is made first to pass through a polarization control device that gives it the required polarization status and is then launched onto the fiber by means of a polarization-insensitive directional coupler. This unit receives the light emitted by the far end of the fiber and sends it to a polarization analyzer connected to a processing device that performs the processing necessary to achieve polarization dispersion.
Another polarization dispersion analysis apparatus based on light reflected from the fiber end is described in WO-A-9836256 by A. Galtarossa. This apparatus is however based on the principles of time-dependent optical reflectometry and is conceived to measure polarization dispersion spatial distribution rather than its average value along the whole length of the connection.
When performing measurements accessing only one end of the fiber, light reflected at the launch end must be eliminated as it would add to the light reflected by the far end and alter measurements. The article by the inventors mentioned above suggested using an angle connector for this purpose, to launch the light. The use of an angle connector is a solution suitable for laboratory testing but not for field measurements as it is not commonly applied in field systems.
The object of the invention is to provide a process and an apparatus for measuring polarization dispersion based on backreflected light analysis, which can be easily used on already installed systems.
The process covered by the invention provides for sending to a nearby end of the fiber measurement light at different wavelengths with a plurality of polarity conditions for each wavelength, collection of the corresponding light reflected by the remote end of the fiber and leaving through the said near end, and polarization analysis and processing of the polarization analysis results. It is characterized by the fact that during transmission of measurement light in the fiber, the reflected light from such near end is masked to prevent it from reaching the facilities performing the polarization analysis and overlapping such reflected light from the far end.
The apparatus for achieving the process consists of:
a wavelength tuneable source to subsequently generate different wavelengths included within a preset wavelength interval;
polarization control means to supply a sequence of polarization conditions to the light emitted by the source for each of such wavelengths;
means for launching into a near end of the fiber the light from the polarization control means and collecting the light reflected by a remote end of the fiber itself and leaving such near end;
means for analyzing polarization, receiving light collected by the launch and collection means; and
processing and control means to perform the processing required for obtaining the value of polarization dispersion from the information supplied by the polarization analysis means and to check the polarization analysis means and the polarization control means,
and is characterized by the fact that the measurement light launch means and backreflected light collection means include means for masking light reflected by the near end of the fiber during launch to prevent it from overlapping with light reflected by the remote end of the fiber in the polarization analysis means.